Gyroscopes are commonly used as sensors for measuring angular velocity in many applications including navigation and guidance, as well as for control stabilization. Although, conventional rotating wheel, fiber optic and ring laser gyroscopes have dominated a wide range of applications, they are too large and often too expensive to be used in emerging applications. Navigation and guidance systems generally utilize gyroscopes together with accelerometers.
FIG. 1 is a block diagram of a known inertial reference unit (IRU) 100. IRU 100 includes a sensor unit 112 which typically includes one or more of gyroscopes and accelerometers which provide inertial signals 114 to processor 116. Sensor unit 112 is generally referred to herein as a primary sensor unit. Processor 116 is programmed to take inertial signals 114 from processor 116 and output inertial data 118 to input/output (I/O) unit 120. I/O unit 120 forwards inertial data 118 to an output bus 122, which is connected to a connector 124 of IRU 100, thereby providing inertial data 118 to other systems within an aircraft, including a display presented to a pilot of the aircraft. As used herein, inertial data generally includes attitude data (e.g. pitch, roll, and heading of the aircraft).
Other functional interfaces are provided at connector 124 of IRU 100, including, but not limited to, an input data bus 130, control discretes 132, status discretes 134, and input power 136. Input power 136 is routed to an internal power supply 140 which generates the specific power requirements for each of sensor unit 112, processor 116, and input/output (I/O) unit 120.
IRU 100 is generally housed in a chassis (not shown), which conforms to particular form factor requirements. The chassis and mounting apparatus includes features for securing IRU 100 in accurate alignment with the aircraft body thereby providing an attitude reference with respect to the aircraft body. Examples of form factors for known IRUs, such as IRU 100, include four MCU and ten MCU. MCU Stands for Modular Concept Unit, which is an industry standard for air transport avionics. A ten MCU chassis is about 12.69 inches wide, 12.5 inches deep and about 7.64 inches high.
Recent advances in micro-machining technology have made the design and fabrication of Micro-Electro-Mechanical Systems (MEMS) sensors including MEMS gyroscopes and accelerometers possible which provide the required small size. Regarding gyroscopes, MEMS devices are several orders of magnitude smaller than conventional mechanical gyroscopes, and can be fabricated in large quantities by conventional batch semiconductor processing. Thus, there is great potential to significantly fabrication cost. The emergence of MEMS gyroscopes is opening up new market opportunities and applications in the area of low-cost to medium performance inertial devices.
Most MEMS gyroscopes are laminar vibratory mechanical structures fabricated on polysilicon or single crystal silicon. Common fabrication steps include bulk micromachining, wafer to-wafer bonding, surface micromachining, and high aspect ratio micromachining. Each of these fabrication steps involves multiple process steps such as deposition, etching and patterning of various materials.
Conventional gyroscopes are based light propagation which utilize the Sagnac effect. The Sagnac effect is used in an arrangement referred to as a ring interferometry or a Sagnac interferometer. In such devices, a beam of light is split and the two resulting light beams are made to follow a trajectory in opposite directions. To act as a ring the trajectory must enclose an area. On return to the point of entry the light is allowed to exit the apparatus in such a way that an interference pattern is obtained. The position of the interference fringes is dependent on angular velocity of the setup.
Usually several mirrors are used, so that the light beams follow a triangular or square trajectory. Fiber optics can also be employed to guide the light. The ring interferometer is located on a platform that can rotate. When the platform is rotating the lines of the interference pattern are displaced sideways as compared to the position of the interference pattern when the platform is not rotating. The amount of displacement is proportional to the angular velocity of the rotating platform. The axis of rotation does not have to be inside the enclosed area.
When the platform is rotating, the point of entry/exit moves during the transit time of the light. So one beam has covered less distance than the other beam. This creates the shift in the interference pattern. Therefore the interference pattern obtained at each angular velocity of the platform features a different phase-shift particular to that angular velocity.
The Sagnac effect is the electromagnetic counterpart of the dynamics of rotation. A gyroscope that can move around freely in a mounting can be used to measure the rotation of the mounting, and likewise, a Sagnac interferometer measures its angular velocity with respect to the local inertial frame.
Although light-based Sagnac gyroscopes are suitable for certain applications, interfacing such interferometers with detectors and signal processors requires transduction which adds to system complexity and cost, and can also add errors. Moreover, the inherent sensitivity provided by conventional Sagnac gyroscopes is not high enough for certain applications, such as high performance inertial measurement units (IMUs) needed for advanced navigation and guidance applications.